Th17 cells have emerged as potent inducers of autoimmune diseases. However, discovery of the mechanisms by which Th17 cells fulfill their pathogenic functions remains elusive. We have found that Th17 cells migrate quickly and accumulate in the CNS of C57BL/6 mice immunized with MOG35-55 prior to the onset of Experimental Autoimmune Encephalomyelitis (EAE), an animal model of MS, but decline rapidly thereafter when Th1 numbers are maintained. This raises the possibility that Th17 cells might be necessary for the recruitment of other pathogenic T cells, such as Th1 cells, after the peak of disease to perpetuate EAE progression. We found that DOCK8-deficient T cells differentiate poorly into Th17 cells. More importantly, DOCK8-deficient mice were completely resistant to the development of EAE. DOCK8 is one of 11 members of the DOCK180 family of proteins, some of which have been shown to function as guanine nucleotide exchange factors (GEFs) that bind and activate small GTPases of the Rho/Rac/Cdc42 family. Although it has been suggested that DOCK8 might coordinate cytoskeletal arrangement, cellular detachment and regulate cell migration, the precise role of the DOCK proteins in the cell remains for the most part unknown. It is possible that DOCK8 might act as a scaffolding protein that is important for the activation o unknown factors necessary for the differentiation and trafficking of Th17 cells. DOCK8 is mainly expressed in lymphocyte and myeloid cell subsets such as dendritic cells (DCs). However, to date we do not know the molecular mechanisms by which DOCK8 regulates the differentiation and the function of these various cell subsets. In order to investigate the in vivo role of DOCK8, we have generated a DOCK8-conditional knockout mouse in which DOCK8 expression could be specifically deleted in CD4 T cells, and Th17 cells. Based on our observation that DOCK8-deficient mice are resistant to the development of EAE, we postulate that DOCK8 could be a novel therapeutic target for the treatment of MS. In order to study how DOCK8 expression affects the pathogenesis of EAE, we propose to carry out the following specific aims: Aim 1. Determine the in vivo role of DOCK8-expressing cells in the development of EAE: We postulate, on the basis of preliminary studies, that DOCK8 plays a critical role in development of Th17 cells and their trafficking in vivo. We will use conditional DOCK8 knockout mice to determine how DOCK8 affects the function of Th17 cells in vivo. Aim 2. Determine the role of DOCK8 in T cell signaling: Based on our preliminary data, our working hypothesis is that signaling mediated by DOCK8 overlaps with, and may directly impact, STAT3 activation and function. We will assess in human and mouse T cells how DOCK8 contributes to cytokine-induced STAT3 activation such as IL-6, IL21, IL10 and IL-23.